A blockchain is essentially an Event Sourced system. And Right To Be Forgotton is a common issue in all event sourced systems.
Several technical and one non technical solution are common:
Encrypt events with per actor keys. Delete the key when user wants to be forgotton. Or encrypt just certain attributes in the same way.
Keep actual data outside of events but only include a link to the data and checksum of the data.
Avoid storing PII in the first place. Does your domain model really require an IPaddress or First Name?
All have tradeoffs, none are silver bullets. But it is a solved problem in Event Sourcing, so it is a solved problem with blockchains (if only new programmers new this and reused patters instead of continually reinventing them)
> Or, wait, you're going to centrally store these keys?
No necessarily. Three alternatives are common:
Data is encrypted with a key only the actor owns. Works for cases where that actor is the only one who should access it (I Reveal My Attributes type of schemes)
Data is encrypted with a sum (combination) of the key from an actor plus some other actors have: only you and the recipient can decrypt. Works for cases like messaging or p2p data exchange.
Data is encrypted with a OR of the key from an actor plus one by a trusted proxy: proxy and actor have ability to decrypt. This is closest the "centrally" storing. With the difference that "proxy" allows for a little more flexibity -in theory-.
The blockchain is decentralised though so you can't do things like per actor encryption that can be deleted AFAIK. It requires a separate, mutable source of data.
Similarly storing all the data off the blockchain just means you didn’t need a blockchain.
A blockchain is essentially an Event Sourced system. And Right To Be Forgotton is a common issue in all event sourced systems.
Several technical and one non technical solution are common:
Encrypt events with per actor keys. Delete the key when user wants to be forgotton. Or encrypt just certain attributes in the same way.
Keep actual data outside of events but only include a link to the data and checksum of the data.
Avoid storing PII in the first place. Does your domain model really require an IPaddress or First Name?
All have tradeoffs, none are silver bullets. But it is a solved problem in Event Sourcing, so it is a solved problem with blockchains (if only new programmers new this and reused patters instead of continually reinventing them)